The present invention generally relates to semiconductor devices and methods of producing the same, and more particularly to a semiconductor device having an active layer in which at least a bipolar transistor is formed and a method producing such a semiconductor device.
FIG. 1 shows an illustrative bipolar transistor formed on an insulator layer. In FIG. 1, the bipolar transistor has a silicon dioxide (SiO.sub.2) layer 100, a silicon (Si) active layer 101, an n.sup.+ -type buried collector region 102, an n.sup.- -type region 103, a p-type base region 104, an n.sup.+ -type emitter region 105, and an n.sup.+ -type collector wall region 106. In such an npn bipolar transistor, a portion of a collector electrode is made of the n.sup.+ -type buried collector region 102 having a high impurity density. For this reason, a surface area of the collector electrode is large and a stray capacitance of the collector has a value which cannot be neglected, thereby limiting the switching speed of an element. In addition, because the n.sup.- -type region 103 is provided on the n.sup.+ -type buried collector region 102, in order to make a breakdown voltage between the base and the collector high it is essential that the production process includes an epitaxial growth process and this leads to a complex production process and a high production cost.
There are cases where a metal oxide semiconductor (MOS) element is formed in the same Si active layer 101 together with the bipolar transistor of the type described above. In such cases, the thickness of the Si active layer 101 should be set to a value in the range of 1 micron in order to realize a high-speed operation of the MOS element; however, the Si active layer 101 actually has a thickness in a range of 5 microns, for example, because of structural reasons of the bipolar transistor. In other words, if the Si active layer 101 is made extremely thin, it becomes impossible to form the bipolar transistor in the Si active layer 101.
Accordingly, it is possible to consider forming a lateral bipolar transistor having a lateral structure in the thin active layer. FIG. 2 shows an illustrative lateral bipolar transistor having an SiO.sub.2 layer 110, an Si active layer 111, an n.sup.+ -type emitter region 112, a p-type base region 113, an n.sup.+ -type collector region 114, and a p.sup.+ -type emitter electrode 115. Such a lateral npn bipolar transistor has an advantage in that the lateral npn bipolar transistor can be formed in an extremely thin Si active layer 111, but suffers disadvantages in that the amplification is small and the breakdown voltage between the base and the collector is low.
On the other hand, when a trench isolation 120 is formed, as shown in FIG. 1 for the purpose of isolating elements, it has been confirmed that a large amount of lattice defects in the region 121 is generated in a vicinity of the trench isolation 120 because of the high impurity density of the n.sup.+ -type buried collector region 102. It is desirable that such lattice defects 121 are prevented from the point of view of improving the reliability of the semiconductor device. In addition, it is necessary to separate the collector region 102 and the trench isolation 120 by a certain distance horizontally in FIG. 1 in order to avoid the undesirable effects of the lattice defects 121, and there is a limit to improving the integration density of the semiconductor device.
Therefore, in the above described illustrative semiconductor devices in which at least a bipolar transistor is formed in an active layer, there are problems in that the surface area of the collector electrode is large and the stray capacitance of the collector has a value which cannot be neglected, thereby limiting the switching speed of the element. In addition, because of the structure of the bipolar transistor, it is impossible to make the active layer extremely thin. Furthermore, there are also problems in that the production process must include the epitaxial growth process which makes the production process complex and the production cost high.